Investments (Investment Income Net of Related Expenses) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	$ 596,035	$ 576,317	$ 1,684,151	$ 1,646,934
Investment expense	(23,293)	(19,399)	(67,019)	(57,114)
Investment income, net of related expenses	572,742	556,918	1,617,132	1,589,820
Available-for-sale Securities [Member]
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	378,669	359,157	1,121,496	1,039,392
Equity securities
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	1,319	942	3,710	3,296
Mortgage Loans on Real Estate [Member]
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	54,424	50,040	155,083	138,829
Policy loans
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	14,730	15,404	44,285	45,870
Funds withheld at interest
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	108,232	102,144	270,094	327,089
Short-term Investments [Member]
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	3,067	1,977	9,276	5,266
Other invested assets
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	$ 35,594	$ 46,653	$ 80,207	$ 87,192